Checking Time. Length of a Saturn Day Revised Again

About 30 years ago, the Voyager spacecraft flew past Saturn. At the time, it measured a Saturn day at 10 hours and 39 minutes. The measurement was based on frequent changes of Saturn’s radio waves detected by Voyager.

In 2004, the Cassini spacecraft found Saturn’s day was a little longer at 10 hours and 47 minutes. It turns out, both numbers were wrong. According to a new report this week, a Saturn day lasts 10 hours and 32 minutes and 45 seconds – give or take 46 seconds.

Why is nailing down the length of a Saturn day so hard? First let’s take a look at how a gas giant’s day is usually figured out. ScienceMag’s Ken Croswell explains how Jupiter’s day is calculated.

No one disputes Jupiter’s spin rate of 9 hours and 55 minutes, because deep beneath that planet’s atmosphere, its core generates a magnetic field, the axis of which is tilted compared with the spin axis; therefore, as the planet turns, the magnetic field sweeps around like the beam of a lighthouse, sending out radio waves that make it easy to measure how fast the hidden core spins.

The same should work for Saturn right? The problem with Saturn is that its magnetic and spin axes are aligned.

Ravit Helled and her fellow scientists figured out the spin from Saturn’s gravity field. Saturn’s spin is fast enough to make its equator stick out. This warps the gravity field. By measuring the gravity field, the scientists were able to come up with this new time.

Now, this time isn’t set in stone. Here’s a part of the study’s abstract (emphasis mine).

Here we report a period of 10 h 32 min 45 s ± 46 s, based upon an optimization approach using Saturn’s measured gravitational field and limits on the observed shape and possible internal density profiles. Moreover, even when solely using the constraints from its gravitational field, the rotation period can be inferred with a precision of several minutes.

Helled and the other scientists double checked their method by measuring Jupiter’s rotation period. Their method lined up with the well-known spin rate of Jupiter.

The new number also changes what we know about Saturn’s winds according to Croswell. The old number suggested Saturn’s winds blow in one direction. The new number would mean half of the planet’s winds blow east and the other half blow west.

The Latest From Cassini

The Cassini spacecraft continues its mission around Saturn and its moons. Earlier this month, Cassini detected tiny grains of rock that point to hydrothermal activity on the seafloor of Saturn’s moon Enceladus.

It’s another find that adds to the possibility that Enceladus could have environments suitable for some type of living organism.

Image credit: NASA. The false-color picture was taken by Cassini using a several spectral filters sensitive to wavelengths of near-infrared light. The picture is of Saturn’s north pole.